This invention relates to arrays of photosensitive pixels, for example for use in image sensors. The invention is particularly concerned with array configurations which enable a reduction in the size of image sensor pixels, so that an image sensor having an increased resolution may be produced.
Various pixel configurations are known which may be used to form a large area photosensitive array. Generally, the photosensitive pixel of a large area array comprises a light-collecting device, such as a photodiode, and a switching device such as a light-shielded diode. In this arrangement, the charge stored on the capacitance of the photodiode is used to provide an indication of the level of light previously incident on the photosensitive pixel.
A known alternative pixel configuration is one in which the photodiode acts as the switching device, and a separate capacitor is required for charge storage. U.S. Pat. No. 4,797,560 discloses an array of photosensitive pixels in which each pixel comprises a photodiode and a capacitor in series, disposed one above the other over a common substrate. The capacitor and photodiode of each pixel comprise thin film layers disposed over the common substrate. Each pixel in U.S. Pat. No. 4,797,560 comprises a single stack of thin film layers, and this enables the stack to occupy a small area (because the photodiode can occupy almost the full area of the pixel) enabling an increased resolution of an image sensor using the pixel configuration. However, the alignment of the pixel stack over previously formed conductors is critical, and the capacitance of each pixel is dependent on this alignment.